1. Field of the Invention
This invention relates in general to offshore drilling and production equipment, and in particular to a subsea well system for monitoring the pressure in a non-producing string of casing through the completion system.
2. Description of the Prior Art
A subsea well that is capable of producing oil or gas will have a conductor housing secured to a string of conductor pipe which extends some short depth into the well. A wellhead housing lands in the conductor housing. The wellhead housing is secured to an outer or first string of casing, which extends through the conductor to a deeper depth into the well. Depending on the particular conditions of the geological strata above the target zone (typically, either an oil or gas producing zone or a fluid injection zone), one or more additional casing strings will extend through the outer string of casing to increasing depths in the well until the well is cased to the final depth. Each string of casing is supported at the upper end by a casing hanger. The casing hanger lands in and is supported by the wellhead housing.
In some shallow wells and in some fluid injection wells, only one string of casing is set within the outer casing. Where only one string of casing is set within the outer casing, only one casing hanger, the production casing hanger, is landed in the wellhead housing. In this case, the space between the outer or first string of casing and the second or production string of casing is isolated by a casing hanger packoff that seals between the wellhead housing and the production casing hanger.
The more typical case is where multiple strings of casing are suspended within the wellhead housing to achieve the structural support for the well to the depth of the target zone. Where multiple strings of casing must be set within the outer casing, multiple casing hangers are landed in the wellhead housing, each set above the previous one in the wellhead housing. Between each casing hanger and the wellhead housing, a casing hanger packoff is set to isolate each annular space between strings of casing. The last string of casing extends into the well to the final depth, this being the production casing. The strings of casing between the outer casing and the production casing are intermediate casing strings.
When drilling and running strings of casing in the well, it is critical that the operator maintains pressure control of the well. This is accomplished by establishing a column of fluid with predetermined fluid density inside the well. During drilling operations, this fluid is circulated down into the well through the inside of the drillstring out the bottom of the drillstring and back to the surface. This column of density-controlled fluid balances the downhole pressure in the well. When setting casing, the casing is run into the pressure balanced well. A blowout preventer system is employed during drilling and running strings of casing in the well as a further safety system to ensure that the operator maintains pressure control of the well. The blowout preventer system is located above the wellhead housing by running it on drilling riser to the wellhead housing.
When each string of casing is suspended from the casing hanger in the wellhead housing, a cement slurry is flowed through the inside of the casing, out of the bottom of the casing, and back up the outside of the casing to a predetermined point. An open fluid communication passage in the casing hanger leading from the casing annulus to the casing interior would adversely affect the flow path of the cement slurry. This could also cause well pressure control problems for the operator under certain conditions.
In a subsea well capable of producing oil or gas, the production fluids flow through perforations made in the, production casing at the producing zone. A string of tubing extends to the producing zone within the production casing to provide a pressure-controlled conduit through which the well fluids are produced. At some point above the producing zone, a packer seals the space between the production casing and the tubing to ensure that the well fluids flow through the tubing to the surface. The tubing is supported by a tubing hanger assembly that lands and locks above the production casing hanger, either in the wellhead housing, in a tubing hanger spool or in a horizontal or spool tree (further described below).
Subsea wells capable of producing oil or gas can be completed with various arrangements of the production control valves in an assembly generally known as a tree. Trees with the arrangement of production control valves located vertically above and in line with the production tubing are generally called christmas trees. Trees with the arrangement of production control valves offset from the production tubing are generally called horizontal or spool trees.
For wells completed with a christmas tree, the tubing hanger assembly lands in the wellhead housing above the production casing hanger. Alternatively, the tubing hanger assembly lands in a tubing hanger spool, which tubing hanger spool is landed and locked to the wellhead housing. For wells completed with a horizontal or spool tree, the horizontal tree locks and seals on the wellhead housing. A tubing hanger assembly locks and seals in the horizontal tree. When either a tubing hanger spool or horizontal tree is located on the wellhead housing, the blowout preventer system is landed on the tubing hanger spool or horizontal tree, respectively.
The tubing hanger assembly in each of the above subsea well systems normally has a flow passage for communication with the annulus surrounding the tubing. This passage allows for monitoring pressure above the packer between the interior of the production casing and the interior of the tubing. In some cases the well can also be produced through this annulus flow passage. Virtually all producing wells monitor pressure in the annulus flow passage between the interior of the production casing and the interior of the tubing.
A sealed annulus locates between the production casing and the next larger string of casing. Normally there should be no pressure in the annulus between the production casing and the next larger string of casing, because the annular space between the production casing and the next larger string of casing is ordinarily cemented at its lower end and sealed with a packoff at the production casing hanger end. Pressure build up in the annulus between the production casing and the next larger string of casing could collapse a portion of the production casing, compromising the structural and pressure integrity of the well. Monitoring pressure in the annulus between the production casing and the next larger string of casing of a subsea well is shown in patents, however, it is not done commercially to applicant""s knowledge. Improvements are desired.
In a subsea well with a tree assembly including either a tubing hanger spool or a horizontal tree, the annulus pressure between the production casing and the next larger string of casing is monitored through communication passages external to the tubing hanger. A communication passage extends through the production casing hanger from the exterior of the production casing hanger below the casing hanger packoff to an outlet in the interior of the production casing hanger. A port closure sleeve threads to the interior of the production casing hanger.
The port closure sleeve seals on both sides of the communication passage outlet in the interior of the production casing hanger. With the port closure sleeve located as described, the communication passage between the exterior of the production casing hanger and the bore of the production casing is isolated. The port closure sleeve as designed can be removed after the tree assembly is installed. After the tree assembly is installed, a lower end of a tubing hanger orientation sleeve mates in the interior of the production casing hanger. The tubing hanger orientation sleeve seals on its exterior surface with the interior of the production casing hanger at a point below the communication passage outlet in the interior of the production casing hanger. The tubing hanger orientation sleeve lands in the tree assembly. The tubing hanger orientation sleeve seals on its exterior surface with the interior of the tree assembly. A space between the tubing hanger orientation sleeve and the wellhead housing is created through which casing annulus pressure can communicate with a communication passage in the tree assembly.
The communication passages communicate pressure in the annulus of the production casing to the exterior of the tree assembly. A communication line extends to monitoring equipment at the surface for monitoring the pressure in the annulus of the production as described.
In one embodiment, the tree assembly includes as part of its assembly a horizontal tree. The horizontal tree lands on the wellhead housing. A tubing hanger orientation sleeve lands in the horizontal tree and mates to the interior of the production casing hanger. The tubing hanger orientation sleeve isolates a space between the exterior of the tubing hanger orientation sleeve and the interior of the wellhead housing to link the communication passage in the production casing hanger with the communication passage in the horizontal tree.
In another embodiment, the tree assembly includes as part of its assembly a tubing hanger spool. The tubing hanger spool lands on the wellhead housing, with the tree mounted to the upper end of the tubing hanger spool. The tubing hanger orientation sleeve lands in the tubing hanger spool and mates to the interior of the production casing hanger. In this configuration, the tubing hanger orientation sleeve is not necessarily oriented to the tubing hanger spool, although it may be. The tubing hanger orientation sleeve isolates a space between the exterior of the tubing hanger orientation sleeve and the interior of the wellhead housing to link the communication passage in the production casing hanger with the communication passage in the tubing hanger spool. The communication passage in the tubing hanger spool communicates pressure to the exterior of the tubing hanger spool. A communication line extends to monitoring equipment at the surface for monitoring the pressure.